Solar heated air is an attractive means of home space heating since the hardware components can be made at moderate costs and and there are no system winter time freezeups and major heat transfer problems involved with hot air arrangements.
There are a number of solar heated air systems on the market which are generally similar to solar hot water flat plate collectors in regard to size and shape, but with larger internal volumes so that relatively large volumes of air may be heated and transferred into the home or building. The various types range from passive, thermosyphoning window units for southern window exposures to all-transparent plastic air bags with top lens like ribs.
The more conventional active type units consist of sealed, flat plate construction with continuous air passages within the area, with a blackened absorber structure utilized to absorb solar heat.
While the passive type of solar hot air heater is relatively inexpensive to construct they can usually heat only a single room or local volume within a home or building due to the lack of forced air movement from the unit. The active types of solar hot air units or systems are generally superior to the passive types in terms of the capacity to heat large house volumes and produce nearly uniform temperatures throughout a building, and are particularly cost/effective when connected to existing hot air duct installations in the home or building.
A shortcoming for some solar hot air systems is their lack of effective heat storage means over an extended number of non-sunlit days, with solar heated hot water storage arrangements being superior in this respect compared to hot air storage means. The so-called solar storage batteries are an excellent means for storing solar heated air since the uniform rows of elongated vertical cylinders provide a relatively large surface area within a restricted space for both storing and releasing solar heated air.
In a properly designed solar hot air heating system with a built in rock storage bin in the basement or sub-basement, adequate heat storage over several non-sunlit days can be provided. The better systems provide additional duct runs so that the home can be heated directly from the solar collector, or the heat from the collector can be stored for future use. Such a versatile arrangement is ideal but usually requires a large rock storage bin volume to be effective over three to four non-sunlit days.
These ideal solar hot air heating systems are not economically feasible for the majority of existing homes where difficult excavating for a large volume rock bin would prove to be too costly, so that some other approach must be taken towards acceptable solar heating retrofit arrangements. A solar hot water heating system may be acceptable for many homes since the water heat exchangers are smaller and can be installed adjacent to existing oil or gas burners.
Where homes have sizable grounds or back yards, the installation of an A-frame type of hot air heating and storage unit may provide a cost/effective means of including solar space heating for many homes. The A-frame type of unit is a significant development in solar hot air heating because of its basic simplicity an and effective operation, and is ideal for backyard installations where the solar conversion means cannot be retrofitted to the existing home or building.
A key advantage to the exterior yard/detached A frame unit is that it utilizes both direct and reflected solar rays to heat the air within the fully insulated A-frame housing interior. The front reflective panel is hinged at the front base so that it can be folded up during the summer months, when heated air is not normally required, but provides full area solar reflection into the A frame housing during the winter months when effective solar heating is most needed. The combination of both equal direct and reflected solar ray transmission into one solar collection volume is a distinctly attractive/cost-effective approach to solar hot air heating systems because of the moderate cost of all the component elements involved.
An important component within the A-frame unit is the front blackened absorber plate which directly receives the solar rays through the double-glazed front windows. This aluminum absorber plate must be made in multi-layers or labyrinth/corregated form to provide the greatest possible surface area within a limited window area.
The A-frame unit is an attractive solar collection method for detached backyard installation where space is available and the unit can be placed facing a true south axis, at an optimium, or mean slope for the latitude of the installation site. Solar hot air heating arrangements offer several advantages over corresponding hot water systems including generally lower installation costs and the elimination of anti-freezeup provisions. For homes or buildings with existing hot air ducts, such solar hot air heating arrangements should be given first choice consideration when the question of adding solar energy conversion for a home arises.
Although the hot air ducts, to and from the home must be dug in slightly below the ground surface and insulation added, this necessity does not usually present major installation problems. All of the hot air ducts should have relatively large radius turns as required, so that the hot air flow is not snubbed or impeded as it moves along ducts which would impose unnecessary loading on the air blowers.
The present rock storage bins used in solar planned houses are not entirely satisfactory, since large storage volumes are required which usually precludes their retrofitting into most existing homes. The eutectic salts--(Gluber's salts) heat storage means is an ideal medium, but somewhat costly for reduced storage volumes for retrofitting solar hot air heating systems, so that alternative material and methods may be sought for the backyard installations. Eutectic salts are a more active means of storing and releasing solar heat than rock storage bins and can occupy considerably less space than a corresponding rock bin. The eutectic salts undergo a change of state from a solid to a liquid when solar heat is absorbed. They remain at a uniform, stable temperature as the solar heat is gradually released and return to their normal solid state condition.
The basic A-frame unit and the principle of full area solar reflection can be expanded to include solar ray reflection into the top and bottom of a suspended type of double-glazed and insulateed solar collector. This suspended, or elevated type of solar hot air collector will produce very high temperatures within the housing which can be stored to produce useful home heating over above four to five days to overcast, winter days. An A-frame type of unit in this suspended form can be an advantageous addition to flat roof top homes or buildings, since it can be aligned on a true south axis at the correct solar slope for the latitude of the installation site.
The suspended, hot box type of solar hot air collector can also serve as a heat storage unit for additional hot air linear solar trough concentrators which are directly connected, in-line with each hot box storage unit. When connected in this way there is no longer any need for some of the extra front reflective surfaces for the suspended type of hot box unit and it may be modified accordingly. The suspended heat storage unit will contain a maximum volume of eutectic salt trays which are all connected to the front absorber plate. The solar heated air from the linear solar concentrators will be blown into the hot box storage unit(s) by multiple fans within the ducts of the linear concentrator sections.